The goal when using an equalizer is to either boost, cut or attenuate certain audio frequencies of sounds to give them depth and space in a mix,

emphasize certain characteristics of sounds so they cut through the mix better, isolate or remove specific frequencies, and

reduce muddiness with competing frequency ranges to achieve clarity.

Here are two wonderfully short and succinct articles which will give you all the tools you need to master EQ:

Warm a voice and improve clarity

5 EQ Tips for Filmmakers

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